Supports for securing the head of a patient during surgical or radiological procedures are known in the art. Such supports are typically adjustable so the head of the patient may be secured in different positions for different radiological views, or to facilitate access to a patient's head during a surgical procedure. Such head supports typically include a base unit that mounts to the head end of a patient support table. The base unit has a horizontal cross bar that supports one or more adapters, which in turn support a skull clamp that holds the patient's head. The skull clamp and the one or more intervening adapters or other members enable operating room attendants to adjust the height, distance, and orientation of the skull clamp with respect to the end of the table, to hold the patient's head in a desired position. An exemplary support system includes a base unit, a skull clamp, and a swivel adapter.
Many skull clamps on the market use a two piece U-shaped construction with one end of the U-shape holding one inwardly directed pin and the other end holding a rocker arm that supports two spaced head holding pins. The two pieces are referred to in this disclosure as the base piece and the ratchet piece. At the closed end of the U-shape, the ratchet piece has a ratchet arm that slides through a slot or passage formed in the base piece. A locking mechanism mounted to the base piece relative to the slot engages the ratchet aim to prevent it from backing out. The slot through the base piece is only accessible from either end. Because the slot is several inches long and only wide enough to accept the complementary shaped ratchet arm, visual access to the interior surfaces of the slot or to the ratchet locking mechanism is limited. Thus, the shape and the interconnection of the slot and the ratchet arm limit the ability to clean or to inspect the slot. Yet, the use of such a skull clamp in a surgical environment practically assures that fluids of one type or another, or other substances, will inevitably find their way to the inside surfaces of the slot. Thus, there is a need for a design that facilitates cleaning and inspection of the inside surfaces of the slot and the locking mechanism.
Typically, head supports such as skull clamps have been fabricated from stainless steel or other metals. However, head supports made of metal components are radiopaque to x-rays, and thus produce artifacts in the x-rays taken when the skull clamp is interposed between the x-ray source and the x-ray film. These artifacts are created by the metal components, which block the path of the x-rays as they travel from the source to the film. More particularly, the areas of the x-ray film where the x-rays are blocked by the metal will be unexposed, and will appear as artifacts in the developed x-ray picture. These artifacts diminish the usefulness of the developed x-ray picture because they obscure the image of a portion of the head that normally would be viewable, absent the obscuring metal portion of the head support.
For these reasons, head supports can be made from radiolucent materials that permit x-rays to pass therethrough, thereby to reduce artifacts. An example of a known radiolucent skull clamp is shown in the Day, et al. U.S. Pat. No. 5,276,927, owned by the assignee of this application. This '927 patent discloses a radiolucent head support with a radiolucent skull clamp secured to the head support. When using this structure, the radiolucent material results in fewer artifacts in the developed x-ray picture.
Another example of a known radiolucent skull clamp is shown in Dinkier, U.S. Pat. No. 5,537,704, also assigned to the assignee of this application. Similar to the skull clamp structure described above, it discloses a radiolucent head clamp having a U-shaped frame with a fixed head-engaging pin at one open end of the U-shape and a pair of head-engaging pins on the opposite end of the U-shape. The closed end of the U-shape of the clamp has a radiolucent rotation mechanism for adjusting the angular position of the pair of head-engaging pins and a radiolucent translation mechanism for linearly moving the pair of head-engaging pins with respect to the fixed head-engaging pin.
Generally, the skull clamps currently on the market use a ratchet and pawl mechanism to control the position of the ratchet arm relative to the base piece. More particularly, ratchet teeth along the ratchet arm engage a pawl housed inside the slot of the base piece. The pawl is biased into engagement with the ratchet teeth, so that once a ratchet tooth moves past the pawl it cannot be moved in the other direction, until a biased plunger mounted on the base piece is pulled against its biasing force to move the pawl away from the ratchet teeth, thereby to allow the ratchet arm to be removed from the slot. The distance between adjacently located ratchet teeth is called the pitch. As the ratchet arm moves further into the slot, the pawl engages another ratchet tooth each time the ratchet arm travels into the slot a distance of one complete ratchet tooth, i.e., the distance of the pitch. A neurosurgical skull clamp device that uses a sliding ratchet mechanism and single pawl is described in U.S. Pat. No. 3,835,861. Each of the three above-cited patents is incorporated by reference herein, in its entirety.
With this type of structure, the distance between ratchet teeth should be sufficient enough to provide robust teeth, so as to assure a strong connection for the head holding components. However, the larger the teeth, the greater the pitch (distance between two ratchet teeth), and the greater the pitch, the lower the adjustability of the ratchet piece relative to the base piece.
Greater adjustability could be achieved with finer teeth. However, finer teeth would likely be more readily damaged over the course of normal use. And for some materials, including the plastic-type radiolucent materials used to make a radiolucent skull clamp, finer teeth would be structurally susceptible to breakage.
Thus, while the x-ray pictures produced by radiolucent head supports have fewer artifacts, such head supports can present other problems. For instance, compared to metals, radiolucent materials are not as well suited for certain components in the adjusting mechanisms of the above described skull clamp design. If such components are made from a plastic-type radiolucent material, the clamping forces could be reduced over time, because the plastic radiolucent components can be susceptible to deformation, and can lose their initial geometry. These conditions could eventually result, over time, in a loss of dimensional precision for the clamp. As a result, the operation and fixation of the two skull clamp frame pieces, and hence the positioning of the patient's head, could become inconsistent and unsatisfactory.
Thus, there is a need to assure a long-lasting, consistent and robust connection of the two pieces of a skull clamp, particularly for a skull clamp that is made from radiolucent materials.